6. Connecting WIDE SCA-2 (80 pin) drives to a NARROW bus (50 pin).
A narrow bus by definition is 50 conductors, but may be either Single Ended (SE) Low Voltage Differential (LVD) or differential (HVD). HVD devices cannot be mixed SE or LVD devices on the same bus. A wide drive can only be added to a narrow bus if the bus is Single Ended (SE).
Check for the presence of term power on pin 26 of the bus, provided by the host, any drive or the backplane.
An adapter is required to convert the SCA-2 80-pin interface to the narrow 50-pin bus connector. The upper byte, or ‘high byte’ of the wide connector, must be terminated with +5volts inside the adapter, because a floating (not connected) data line is an undefined signal level and may cause parity errors. The DIFFSENS line, pin 46 on the 80 pin side, should be grounded for SE operation.
Some SCA-2 adapters also provide bus termination; if the drive is not at the end of the bus check that the termination can be disabled on the adapter.
Check for two terminators, one at each physical end of the bus. Many 50 pin drives found on a narrow bus are internally terminated; check jumper settings to enable/disable.
Active terminators are required if Ultra SCSI speeds (20 Mbytes/sec) or higher are being used.
No more than 8 devices are supported in this configuration; see Table 1.
Check maximum bus (cable) length, as described in Table 1.
Check maximum stub length does not exceed 0.1m (4").
Check SCSI IDs. Each device/adapter must have a unique ID, typically set by jumper.
Check for remote start/start delay jumpers and remove if necessary.
Check for ‘Force SE’, ‘Disable Wide’ jumpers and apply as required.
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